Here's where the requirement came from:
Duncan Aviation
If I install WAAS-capable equipment, am I ready to fly LPV approaches?
Questions that we’re often asked are if I install WAAS-capable equipment, am I ready to fly LPV approaches? The answer is no. WAAS receivers cannot be installed under a straight field approval. There’s a lot more to the field approval process. Once it’s installed in the aircraft, the installing agency needs to make sure that all equipment in the airplane is properly functioning. That means the autopilot, the scaling, everything that becomes a part of this equation needs to be checked, so it’s a lot more stringent than a straight field approval. Most WAAS receivers are installed under an STC.
WAAS-capable avionics do not automatically mean that you can fly to an LPV minimum. To accomplish the LPV minimums, you need dual WAAS receivers. They must be certified under TSO 145/146.